Thrombotic Disorders
In a healthy person, a homeostatic balance exists between procoagulant (clotting) forces and anticoagulant and fibrinolytic forces. Numerous genetic, acquired, and environmental factors can tip the balance in favor of coagulation, leading to the pathologic formation of thrombi in veins (e.g. deep vein thrombosis), arteries (e.g. atherothrombosis, myocardial infarction, ischemic stroke), or cardiac chambers. Thrombi can obstruct blood flow at the site of formation or detach and embolize to block a distant blood vessel (e.g. pulmonary embolism, stroke).
Accumulating evidences show that atherothrombosis, a world-leading life-threatening disease, is linked to a defective immunoregulation driving a pathologic activation of blood leukocytes and a destructive inflammatory response within the vascular wall. Consequently, a restoration of immunoregulation at the blood-vessel interface would represent an innovative therapeutic option to fight atherothrombosis.
Autoimmune Disorders
In autoimmune disorders, the immune system produces antibodies to an endogenous antigen. Antibody-coated cells, like any similarly coated foreign particle, activate the complement system, resulting in tissue injury. Autoimmune disorders include systemic lupus erythematosus (SLE), rheumatoid arthritis (RA), multiple sclerosis (MS), inflammatory bowel disease (IBD), Graves' disease and diabetes mellitus.
Several mechanisms may account for the body's attack on itself. Autoantigens may become immunogenic because they are altered chemically, physically, or biologically. Certain chemicals couple with body proteins, making them immunogenic (as in contact dermatitis). Drugs can produce several autoimmune reactions by binding covalently to serum or tissue proteins (see below). Photosensitivity exemplifies physically induced autoallergy: Ultraviolet light alters skin protein, to which the patient becomes allergic. In animal models, persistent infection with an RNA virus that combines with host tissues alters autoantigens biologically, resulting in an autoallergic disorder resembling SLE.
Most human autoimmune diseases are specific antigen-driven T-cell diseases. T-cell clones responding to specific antigenic epitopes are responsible for the initiation and/or the propagation of these diseases. Similarly, specific antigen-driven T-cell responses are responsible for the rejection of organ allografts and the immune response to tumors. Activated T cells provide the “engine” for the chronic inflammation that is associated with autoimmune diseases, organ graft rejection and tumor immunity.
CD31 (PECAM-1)
Immune responses can be controlled by inhibitory immune receptors among which CD31 (PECAM-1), which is expressed exclusively and constitutively on cells at the blood-vessel interface.
CD31 consists of a single chain molecule comprising six Ig-like extracellular domains, a short transmembrane segment and a cytoplasmic tail containing two ImmunoTyrosine-based Inhibitory Motif (ITIM)s. The structure of CD31 is shown in the table below.
PositionDomainon SEQ ID No: 1Signal peptide 1 to 27Extracellular domain 28 to 601First Ig-like extracellular domain 34 to 121Second Ig-like extracellular145 to 233domainThird Ig-like extracellular domain236 to 315Fourth Ig-like extracellular328 to 401domainFifth Ig-like extracellular domain424 to 493Sixth Ig-like extracellular domain499 to 591Juxta-membrane domain592 to 601Transmembrane domain602 to 620Cytoplasmic domain621 to 738
The immunoregulatory properties of CD31 are supported by the fact that CD31 signalling drives mutual repulsion of blood leukocytes and modulates the balance between inhibitory and stimulatory signals of both innate and adaptive immune cells. Mechanical engagement of the distal Ig-like extracellular domains of CD31 induces outside-in inhibitory signalling triggered by the phosphorylation of its ITIMs, and the recruitment and activation of SH2-containing phosphatases.
Zehnder et al. (1995, Blood. 85(5):1282-8) identified a CD31 antibody that inhibited the mixed lymphocyte reaction (MLR) in a specific and dose-dependent manner. They further found that a CD31 peptide corresponding to the epitope of this antibody, i.e. to the 23 membrane-proximal amino acids of CD31, strongly inhibited the MLR. They hypothesized that the 23 membrane-proximal amino acids of CD31 constitutes a functionally important region, and that the CD31 peptide interferes with lymphocyte activation by competing for binding epitopes. However, Zehnder et al. failed to teach whether CD31-mediated signaling is activated or inhibited by the CD31 peptide.
Chen et al. (1997, Blood. 89(4):1452-9) showed that this peptide delayed onset of graft-versus-host disease (GVHD) and increased long-term survival in a murine model of the disease. They hypothesized that the CD31 peptide inhibits a common pathway in T-cell activation. Again, Chen et al. failed to elucidate the role played by the CD31 peptide in T-cell activation. In particular, these previous works did not assess the putative effect of the peptide on the CD31 signaling cascade and more precisely on the phosporylation state of the CD31 ITIMs.
By a yet unknown mechanism, CD31 is “lost” on certain circulating lymphocytes. Its loss is observed upon lymphocyte activation and it has been recently shown that the absence of lymphocyte CD31 signalling, in turn, heightens the pathologic immune responses involved in the development of atherothrombosis.
A soluble form of CD31, due to a variant transcript lacking the transmembrane segment, has also been reported and therefore it is currently thought that the individual amount of circulating CD31 is genetically determined. Consequently, a number of previous studies have attempted to find a correlation between plasma levels of soluble CD31 and the risk of atherothrombosis or other autoimmune diseases. However, independently of the specific genetic polymorphisms analyzed, data showed a broad range of plasma CD31 values and the results of these different studies were contradicting.
There is therefore a need for better understanding the biological function of CD31. This would allow the provision of more efficient therapeutics for the treatment of diseases linked with T-cell activation.